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Understanding the Network: A Practical Guide to Internetworking by Martin J. Michael

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Digital Carrier Systems

Although FDM makes more efficient use of the transport medium's available bandwidth, it has some practical limitations in terms of signal quality and distance. Therefore, in major metropolitan areas, interCO trunks were originally deployed using discrete wire pairs. By the 1960s, a real need arose for a CO-to-CO transmission system that could reduce cable densities and provide reliable transport over longer distances. To meet this requirement, AT&T developed the T-carrier digital transmission system.

Digital transmission is not affected by the line transmission noise and the amplification problems associated with analog transmission, because digital information is transmitted as Binary Coded Decimal (BCD) words instead ...

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